Representative immunofluorescence images of OPN expression in human MSCs seeded on A) tissue culture plate (TCP) (A), flat (B, D), and nanoridged (C, E) SF films cultured in osteogenic induction-free medium (B, C) and osteogenic induction medium (D, E) on Day 14. The cells cultured on the nanoridged SF films (C) with osteogenic induction-free medium expressed more OPN than those cultured on TCP (A) and flat SF films (B) in the osteogenic induction-free medium. The cells cultured on both the flat (D) and nanoridged (E) films expressed OPN in the osteogenic induction medium. OPN was stained by rhodamine-labeled antibody (red) and cell nuclei were stained by 4′, 6-diamidino-2-phenylindole (DAPI) (blue) and F-actin was stained by FITC-labeled phalloidin (green).